The best method for determining the incidence and natural history of a condition.

Prospective cohort studies
A group of people is chosen who do not have the outcome of interest (e.g., myocardial infarction). The investigator then measures a variety of variables that might be relevant to the development of the condition. Over a period of time, the people in the sample are observed to see whether they develop the outcome of interest (that is, myocardial infarction).
In single cohort studies, those people who do not develop the outcome of interest are used as internal controls.
Where two cohorts are used, one group has been exposed to or treated with the agent of interest and the other has not, thereby acting as an external control.

Retrospective cohort studies
These use data already collected for other purposes. The methodology is the same but the study is performed posthoc. The cohort is “followed up” retrospectively. The study period may be many years, but the time to complete the study is only as long as it takes to collate and analyze the data.

Cohort studies: key points
- They describe incidence or natural history.
- They analyze predictors (risk factors), thereby enabling calculation of relative risk.
- They measure events in temporal sequence, thereby distinguishing causes from effects.
- Retrospective cohorts, where available, are cheaper and quicker.
- Confounding variables are the major problem in analyzing cohort studies.
- Subject selection and loss to follow-up is a major potential cause of bias.

See reference for details.

Observational research methods. Research design II: cohort, cross sectional, and case-control studies. CJ Mann. Emerg Med J 2003;20:54–60. Accessed on October 6, 2012.